Process for electrolytic coloring of aluminum cr aluminum alloy articles

ABSTRACT

A process for electrolytic coloring of an aluminum or aluminum alloy article which comprises subjecting the article to an anodic oxidation in an anodic oxidation bath containing essentially sulfuric acid or an aromatic sulfonic acid to form an oxidized film of a thickness of at least 6 microns, immersing the anodic oxidized article as a cathode in an aqueous electrolytic coloring bath containing a water soluble nickel salt and less than 12 ppm of sodium ion, and coloring therein the article using a direct current electrolysis.

United States Patent 1191 Yanagida et a].

1 PROCESS FOR ELECTROLYTIC COLORING OF ALUMINUM CR ALUMINUM ALLOY ARTICLES [75] lnventors: Kiyomi Yanagida; Ta'dashi Hirokane; Tadashi Tsukiyasu; Tomoari Sato, all of Nagoya, Japan [73] Assignee: Sumitomo Chemical Co., Ltd., Osaka, Japan [22] Filed: Mar. 20, 1974 [21] Appl. No.: 453,122

[30] Foreign Application Priority Data Mar. 20, 1973 Japan 48-32027 Nov, 22, 1973 Japan 48-13199 [52] US. Cl 204/35 N; 204/58; 148/627 [51] Int. Cl. C23b 5/52; C23b 9/02 [58] Field of Search 204/35 N, 58; 148/627 [56] References Cited UNITED STATES PATENTS 3.175.963 3/1965 Kessler 204/35 N 3,41 1,994 11/1968 Wainer 204/35 N 3,494,839 2/1970 Chambers et a1 204/35 N [H1 3,891,517 June 24, 1975 3,616,297 10/1971 Cooke et al. 204/35 N 3,616,298 10/1971 Fassell 204/35 N 3,622,471 11/1971 Cooke et :11. 204/35 N 3,761,362 9/1973 Oida et a1. 204/35 N OTHER PUBLICATIONS S. Wernick 84 R. Pinner, The Surface Treatment and Finishing of Aluminum & lts Alloys," (1964), pp. 358-359.

Primary Examiner-John H. Mack Assistant Examiner-Aaron Weisstuch Attorney, Agent, or FirmSughrue, Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT 13 Claims, 2 Drawing Figures SODIUM lON CONTENT (ppm) PATENTEDJUM24 ms 3.891.517

FIG]

U 3 6 9 l2 l5 SODIUM ION CONTENT (ppm) Y 4 X/ /0) x x/x/ POTASSIUM ION comm (ppm) PROCESS FOR ELECTROLYTIC COLORING OF ALUMINUM CR ALUMINUM ALLOY ARTICLES BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to an improved process for coloring an oxidized film on an aluminum or aluminum alloy article (hereinafter referred to, for brevity, as aluminum) in an electrolytic coloring bath containing acids or salts of metals using a direct current electrolysis wherein the aluminum is immersed in the coloring bath as a cathode, and more particularly, to an improvement in the process for coloring an anodic oxi' dized film deep bronze in an electrolytic coloring bath containing water soluble nickel salts.

2. Description of the Prior Art One of the known processes for electrolytically coloring an anodic oxidized film of aluminum is a process involving the anodic oxidation of aluminum in an aqueous solution containing an organic acid.

Another process comprises electrolyzing the preliminarily anodic oxidized aluminum in an electrolytic bath containing a water soluble metallic salt.

Recently, this latter process has been adopted due to R8 eeanomieai advantages and widely applied to produce articles in various areas, for example, to produce fabricating and building materials.

Examples of the latter process include known processes such as an inorganic coloring process as disclosed in US. Pat. No. 3,382,160 using an alternating current electrolysis, a coloring process as disclosed in Japanese Pat. Publication No. 28585/72 using an alternating current treatment and a direct current electrolysis, and a process as described in DT-OLS 2,l 12,927.

When an aqueous solution containing a water soluble nickel salt is used as the electrolytic coloring bath in direct current electrolysis, an amber or bronze color can be provided on the anodic oxidized film. In the production of such colored aluminums on a commercial scale, however, the coloring of the anodic oxidized film is often unstable making it difficult to obtain a deep tone and the resultant colored film often peels off.

SUMMARY OF THE INVENTION After many empirical studies on solving such problems, the inventors have discovered that an anodic oxidized film colored with a stable and uniform deep brown color can be obtained by limiting the sodium ion content in the electrolytic coloring bath to below 12 PP" Therefore, this invention provides a process for coloring an oxidized film on aluminum comprising subjecting the aluminum to an anodic oxidation treatment in an anodic oxidation bath containing essentially sulfuric acid or an aromatic sulfonic acid to form an oxidized film of a thickness of at least 6 microns, immersing the anodic oxidized aluminum as a cathode in an aqueous electrolytic coloring bath containing a water soluble nickel salt and less than 12 ppm of sodium ion, and coloring therein the aluminum using direct current electrolysis.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS FIG. I shows the relation between the lightness Y(percent) of a colored aluminum sample produced according to the process of this invention and the sodium ion content in the coloring bath.

FIG. 2 shows the relation between the lightness Y(percent) of the colored aluminum sample produced according to the process of this invention and the potassium ion content in the coloring bath.

DETAILED DESCRIPTION OF THE INVENTION The advantageous features of the process of this invention, including electrolyzing the preliminarily anodic oxidized aluminum in an aqueous metallic salt solution with direct currect, are as follows.

I. The oxidized film on the aluminum can be colored in extremely short periods of time.

2. The resulting colored film exhibits an attractive uniform tone.

3. The coloring work is easy, and the coloring process of this invention always results in the same tone on the aluminum.

The process of this invention involves the application of a usual anodic oxidized film on aluminum, especially an anodic oxidized film of a thickness of more than 6 microns formed using an anodic oxidation bath containing essentially sulfuric acid or an aromatic sulfonic acid. The sulfuric acid is generally used in a concentration of from 5 to 30 percent by weight in the anodic oxidation bath and optionally, together with a small amount of a carboxylic acid. The aromatic sulfonic acid is generally used in an amount of about l0 percent by weight in the anodic oxidation bath, and it is essential that a small amount of sulfuric acid, e.g., 0.5 to 1.5 percent by weight based on the total amount of the anodic oxidation bath be present together with the aromatic sulfonic acid. Suitable examples of aromatic acids which can be used are sulfosalicylic acid, sulfophthalic acid and the like. Aluminum having such an oxidized film can be colored stably and uniformly and the resulting colored film is quite durable to atmospheric attack.

The oxidized film must have a thickness of at least 4 microns to carry out the coloring treatment, but a deep bronze color is hardly obtained with a thickness of 4 to 5 microns. Accordingly, to achieve this deep bronze color, the thickness of the oxidized film should be at least 6 microns in the process of this invention.

The oxidized aluminum is immersed in the electrolytic coloring bath without applying a sealing treatment and electrolyzed therein using a direct current.

If the sodium ion content in the electrolytic coloring bath containing the water soluble nickel salt exceeds 12 ppm, a deep bronze color can not be obtained even by changing the composition of the coloring bath and the other electrolyzing conditions. Accordingly, the sodium ion content must be restricted to below l2 ppm.

The main component of the electrolytic coloring bath is a water soluble nickel salt, e.g., nickel sulfate, nickel chloride, nickel acetate and the like, preferably nickel sulfate. When nickel sulfate is used, the content is about 10 to 150 g/l, preferably 15 to g/l for producing a favorable colored film. The coloring of the oxidized film can be carried out with a nickel sulfate content varying from this preferred range, but this results in difficulties in the electrolyzing treatment on a commercial scale and thus is economically disadvantageous.

Usually, the electrolyzing coloring bath contains about 10 to 50 g/l of boric acid to obtain a stable and uniformly colored film. If desired, a small amount, for example, less than 1 percent by weight, of sulfuric acid, an organic acid and/or an ammonium salt can be added to the electrolytic coloring bath to control the conductivity and the pH thereof.

A change in the tone of the colored film is possible by adding a small amount of water soluble salts, for example, sulfates or chlorides, of copper, tin, cobalt and- /or iron, such as cuprous or cupric sulfate, cuprous or cupric chloride, stannous sulfate, stannous chloride, cobalt sulfate, cobalt chloride, ferrous sulfate, ferrous chloride and the like, to the coloring bath.

The preferred current density for the electrolyzing coloring treatment is about 0.l to 3.0 A/dm and the preferred electrolyzing time is less than 5 minutes. If the current density is relatively high, a deep tone can be obtained with an electrolyzing time of only 5 seconds. On the other hand, if the current density exceeds 3.0 Aldm the phenomenon in which the oxidized film peels off tends to occur and a stable coloring treatment becomes difficult.

The longer is the coloring time in this process, the deeper becomes the toner of the colored film. With a current density of less than 0.1 A/dm, however, a very long period of time is required to obtain a deep color.

The working temperature of the electrolytic coloring bath is usually ambient temperatures, e.g., room temperature, but may range from about to 40C.

As the sodium ion content in the electrolytic coloring bath increases, the tone of the oxidized film becomes lighter, and if the sodium ion content exceeds 12 ppm, a deep bronze color can not be obtained even by changing the electrolyzing conditions. in addition, the resultant oxidized film often peels off and then the tone is not uniform. Accordingly, the sodium ion content in the coloring bath must be restricted to a value which does not exceed 12 ppm.

The sodium ion content in the coloring bath sometimes exceeds the above upper limit which is believed to be due to the water used for the coloring bath, the impurities contained in the additives for the coloring bath, the atmosphere surrounding the coloring bath, or the liquid carrier over by the aluminum from the pretreatment. Accordingly, the coloring bath must be carefully prepared without contamination by pick-up of sodium ion as shown in the Reference Example given hereinafter.

For example, the pick-up of sodium ion can be avoided by washing the aluminum, the supporting members therefor and the electrolyzing frame with water using a water spraying means between the etching bath and the electrolytic coloring bath and by providing an exhausting device on the etching bath which contains sodium hydroxide. The sodium ion content in the water, in the boric acid and in the nickel salt are measured, and those which show a high sodium ion content must not be used. If possible, deionized water may be used for preparing the coloring bath. in addition, the use of deionized water is preferable for the final washing after the anodic oxidation.

With such treatments for avoiding contamination by sodium ion, large amounts of sodium ion may possibly be introduced into the coloring bath due to misoperation. Therefore, the coloring bath is treated so as to remove the sodium ion therefrom in order to keep the sodium ion content below 12 ppm.

in such a case, the sodium ion must be removed exclusively so as not to change the content of the other components in the coloring bath. The inventors have discovered that the removal of the sodium ion is possible without changing the composition of the coloring bath by using a cation exchange resin wherein nickel is absorbed on the ion exchange group in an ion exchange means, and treating the coloring bath which contains undesirably large amounts of sodium ion with this ion exchange means. According to this treatment for the removal of the sodium ion, the level of the sodium ion in the coloring bath can be remarkably reduced. This treatment is effective not only for relatively high sodium ion contents, e.g., 15 ppm or 25 ppm, but also for relatively low sodium ion contents, e.g., 5 ppm. Accordingly, the sodium ion content in the coloring bath can be greatly decreased by this treatment. The low sodium containing coloring bath emerging from the ion exchange means may be recycled to the electrolytic coloring apparatus for re-use.

The coloring bath may possibly be contaminated by chloride ion, sulfate ion, ammonium ion, potassium ion, ferrous ion, calcium ion, magnesium ion and aluminum ion. Next to the sodium ion, potassium ion influences the properties of the coloring bath to the greatest extent, and its tolerable maximum amoutn is about 20 ppm. Potassium ion, however, is hardly introduced into the coloring bath in such an amount so as to adversely affect the coloring treatment, unless it is intentionally added to the coloring bath.

Other ions do not influence the properties of the coloring bath containing the water soluble nickel salt as compared with the effects of sodium ion and potassium ion. For example, ppm of chloride ion and 500 ppm of aluminum ion do not influence the action of the coloring bath.

As particularly described above, this invention provides an improved aluminum coloring process using an electrolytic coloring bath containing a water soluble nickel salt on a basis of the discovery that the presence of sodium ion in the coloring bath causes an unstable and light tone oxidized film.

Now, more specific embodiments of this invention are described with reference to some Examples thereof, but the scope of this invention is not intended to be construed as being limited to the Examples. Unless otherwise indicated, all parts, percents, ratios and the like are by weight.

EXAMPLE 1 An aluminum sheet (99.2% Al) was first immersed in a 10 percent sodium hydroxide solution at 60C for one minute, and then subjected to a neutralizing treatment with 20 percent nitric acid for 3 minutes at room temperature (about 20 30C).

After washing with water, the sheet obtained was subjected to a direct current electrolysis in a 15 percent sulfuric acid aqueous solution as the anodic oxidation bath with a current density of 2.0 A/dm, for IS minutes at a bath temperature of 20 1- 1C, to form an oxidized film of a thickness of 9 microns. The anodic oxidized aluminum was then immersed in an electrolytic coloring bath containing 35 g/l of nickel sulfate and 35 g/l of boric acid. This electrolytic coloring bath was prepared using chemicals of special grade and deionized water.

The sodium ion content in this coloring bath was almost zero ppm. as the result of a determination using an atomic light absorbing photometer. The aluminum was immersed in the coloring bath as the cathode and subjected to a direct current electrolysis with a current density of 0.5 A/dm, a bath temperature of 20 i 1C for 0.5 minutes using a nickel plate as an anode. After washing with water, the aluminum was subjected to a sealing treatment for 30 minutes in boiling water. Sev eral samples were prepared by anodizing and coloring the aluminum under the same conditions as described above except for using coloring baths containing NaCl to introduce therein 3 ppm of sodium ion, 6 ppm of sodium ion, 9 ppm of sodium ion, 12 ppm of sodium ion and 15 ppm of sodium ion, respectively. The tone of the colored film on each of the samples was observed and compared with each other. Since the change of the tones of the samples was not fully explained, the tone of each of the samples was indicated by the lightness Y (percent) measured using a color difference meter.

FIG. 1 shows the relationship between the lightness Y (percent) of the samples thus obtained and the sodium ion content in the electrolytic coloring bath. The lightness Y (percent) was low with a sodium ion content of less than 6 ppm, and the color of the resultant oxidized film was almost black (0 to 3 ppm) to very deep bronze (3 to 6 ppm). By increasing the sodium ion content from 6 to 12 ppm, the lightness Y (percent) of the oxidized film became high and the color changed from deep bronze to bronze. The upper limit of the sodium ion content to keep the oxidized film deep bronze-colored was about l2 ppm. If the sodium ion content in the coloring bath was higher than 12 ppm, the tone of the oxidized film became light.

FIG. 2 shows the influence of potassium ion in the coloring bath on the tone of the aluminum. The potas sium ion content in the coloring bath was controlled by introducing potassium chloride thereinto. It will be apparent from FIG. 2 that the tone of the oxidized film was scarcely influenced by a potassium ion content of less than 15 ppm but was made light by a potassium ion content of more than ppm.

Thus, sodium ion in the coloring bath greatly influences the tone of the colored oxidized film as compared with potassium ion, both being alkali metal ions. In addition, in a practical coloring treatment on a commercial scale, the chance of contamination by sodium ion is much more frequent than that of potassium ion.

REFERENCE EXAMPLE 400 cc of a strongly acidic cation exchange resin (Na-type) was charged into an exchange column having an inside diameter of 60 mm, and converted from an Na-type into an l-I-type resin by passing hydrochloric acid through the column in a usual manner. An aqueous solution of nickel sulfate having a concentration of 300 g of nickel sulfate per liter of the solution was then passed through the column until substantially all of the H-type resin has been converted into an Nitype resin, that is, until the pH value and the nickel content in the effluent had become substantially identical to those of the nickel sulfate solution to be fed into the column to convert the H-type resin into an Ni-type exchange resin.

Table 1 Amount of Coloring Bath Passed Through Column l Sodium Ion Content in Effiuent (ppm) As is apparent from the above results, the sodium ion in the coloring bath can be removed easily and accurately. That is, the sodium ion content in the coloring bath can be reduced using the above process.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

What is claimed is:

1. In a process for coloring an anodic oxidized film on an aluminum or aluminum alloy article by subjecting said article to anodic oxidation in an aqueous anodic oxidation bath consisting essentially of sulfuric acid or an aromatic sulfonic acid to form an oxidized film of a thickness of at least 6 microns and electrolyzing said anodic oxidized article as a cathode immersed in an aqueous electrolytic coloring bath containing a water soluble nickel salt using a direct current, the improvement which comprises obtaining an anodic oxidized film colored a stable and uniform deep brown which comprises maintaining the sodium ion content in the electrolytic coloring bath below 12 ppm.

2. The process according to claim 1, wherein said water soluble nickel salt is nickel sulfate, nickel chloride or nickel acetate.

3. The process according to claim 2, wherein said water soluble nickel salt is nickel sulfate.

4. The process according to claim I, wherein said electrolyzing of said aluminum is with a current density of 0.] to 3.0 A/dm 5. The process according to claim 1, wherein the content of said sodium ion in said aqueous electrolytic coloring bath is maintained at less than 12 ppm by passing said aqueous electrolytic coloring bath through a column of an Ni-type strongly acidic cation exchange resin to substantially remove sodium ion and recycling the aqueous electrolytic coloring bath having a low sodium ion content.

6. The process according to claim I, wherein said anodic oxidation bath contains said sulfuric acid in an amount of from 5 to 30 percent by weight.

7. The process according to claim 1, wherein said anodic oxidation bath contains said aromatic sulfonic acid in an amount of about IO percent by weight with 0.5 to 1.5 percent by weight of sulfuric acid.

electrolyzing is conducted for a time of less than 5 minutes.

12. The process according to claim ll, wherein the temperature of the electrolytic coloring bath is from about 10 to 40C.

l3. The process according to claim 1, wherein no more than about 20 ppm of potassium ions are present in said coloring bath. 

1. IN A PROCESS FOR COLORING AN ANODIC OXIDIZED FILM ON AN ALUMINUM OR ALUMINUM ALLOY ARTICLE BY SUBJECTING SAID ARTICLE TO ANODIC OXIDATION IN AN AQUEOUS ANODIC OXIDATION BATH CONSISTING ESSENTIALLY OF SULFURIC ACID OR AROMATIC SULFONIC ACID TO FORM AN OXIDIZED FILM OF A THICKNESS OF AT LEAST 6 MICRONS AND ELECTROLYZING SAID ANODIC OXIDIZED ARTICLE AS A CATHODE IMMERSED IN AN AQUEOUS ELECTROLYTIC COLORING BATH CONTAINING A WATER SOLUBLE NICKEL SALT USING A DIRECT DURRENT, THE IMPROVEMENT WHICH COMPRISES OBTAINING AN ANODIC OXIDIZED FILM COLORED A STABLE AND UNIFORM DEEP BROWN WHICH COMPRISES MAINTAINING THE SODIUM ION CONTENT IN THE ELECTROLYTIC COLORING BATH BELOW 12 PPM.
 2. The process according to claim 1, wherein said water soluble nickel salt is nickel sulfate, nickel chloride or nickel acetate.
 3. The process according to claim 2, wherein said water soluble nickel salt is nickel sulfate.
 4. The process according to claim 1, wherein said electrolyzing of said aluminum is with a current density of 0.1 to 3.0 A/dm2.
 5. THE PROCESS ACCORDING TO CLAIM 1, WHEREIN THE CONTENT OF SAID SODIUM ION IN SAID AQUEOUS ELECTROLYTIC COLORING BATH IS MAINTAINED AT LESS THAN 12 PPM BY PASSING SAID AQUEOUS ELECTROLYTIC COLORING BATH THROUGH A COLUMN OF AN NI-TYPE STRONGLY ACIDIC CATION EXCHANGE RESIN TO SUBSTANTIALLY REMOVE SODIUM ION AND RECYCLING THE AQUEOUS ELECTROLYTIC COLORING BATH HAVING A LOW SODIUM ION CONTENT.
 6. The process according to claim 1, wherein said anodic oxidation bath contains said sulfuric acid in an amount of from 5 to 30 percent by weight.
 7. The process according to claim 1, wherein said anodic oxidation bath contains said aromatic sulfonic acid in an amount of about 10 percent by weight with 0.5 to 1.5 percent by weight of sulfuric acid.
 8. The process according to claim 7, wherein said aromatic sulfonic acid is sulfosalicyclic acid or sulfophthalic acid.
 9. The process according to claim 3, wherein from about 10 to 150g/l of said nickel sulfate is present in said electrolytic coloring bath.
 10. The process according to claim 9, wherein from about 10 to 50g/l of boric acid is present in said electrolytic coloring bath.
 11. The process according to claim 4, wherein said electrolyzing is conducted for a time of less than 5 minutes.
 12. The process according to claim 11, wherein the temperature of the electrolytic coloring bath is from about 10* to 40*C.
 13. The process according to claim 1, wherein no more than about 20 ppm of potassium ions are present in said coloring bath. 